(a) Field of the Invention
This invention relates to an agent for separating water from a water-containing system, and more specifically to a water-separating agent having water-absorbing capacity, which varies in degree depending on temperature, is capable of absorbing and retaining water therein and, when heated, is capable of undergoing shrinkage even in the presence of a large excess of water so as to release the thus-retained water.
(b) Description of the Prior Art
Separation of water has been routinely practiced in such process steps as concentration of aqueous solutions, crystallization from aqueous solutions and production of pure water.
As specific techniques useful in the practice of such process steps, may be mentioned inter alia (1) separation of water through such membranes as reverse osmosis membranes and ultrafiltration membranes; and (2) separation of water by making use of phase change of water such as the multi-stage flash distillation method or lyophilizing method. There techniques have already found commercial utility. However, none of these conventional techniques are fully satisfactory. Thus, a variety of improvements has been attempted.
Recently, resins capable of absorbing and retaining water in amounts several hundreds times their own weights have been developed. These resins are generally called higher water-absorbing resins and they are now being applied in various fields. These resins are however accompanied by the following problems:
(1) After absorbing water, the above resins may be regenerated only when heated to drive off the thus-absorbed water. When their regeneration is contemplated for reuse, enormous costs are required.
(2) The amounts of water which the above resins can absorb vary depending on whether the water is pure water or salt water. The amounts will generally decrease in salt water, in some instances to as little as one twentieth of that in pure water.
(3) After absorbing water, the above resins may not be fully satisfactory in dynamic characteristics, especially in shape-retaining capacity.
On the other hand, various physical properties of diverse water-retaining gels have recently been measured. From results of such measurements, it has been found that the water content of water-retaining gels at equilibrium is dependent on the temperature [Journal of polymer Science: Polymer Symposium 66, 209-219 (1979); European Polymer Journal 17, 361-366 (1981); Polymer Bulletin 7, 107-113 (1982)]. It may thus be suggested that there is a chance to develop a water-separating technique making use of such gels under various temperatures. From the practical standpoint, the extent of the swelling of such gels at low temperatures are however not as large as the data given in the above literature. Furthermore, differences in water-absorbing capacity at various temperatures are not satisfactorily large. It has not been investigated how fast such gels would swell or shrink when their temperatures are changed. They are however very unlikely to undergo fast swelling or shrinkage even when their temperatures are changed.
The present inventors have carried out an extensive research with the foregoing in view. As a result, it has been discovered that a water-insolubilized polymer or copolymer of certain specific acryl or methacryl amide derivatives can absorb water and can thus swell to a large degree even at low temperatures, that water-absorbing capacity can vary to a great degree due to temperature changes and that this change in water-absorbing capacity takes place extremely rapidly. Therefore, this water-insolubilized polymer or copolymer has been found to be extremely useful as a water-separating agent from the practical viewpoint.